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Radiation and Air Carrier Crew Members

From the UAL booklet on cosmic radiation, distributed in 2001.

Introduction

The earth is continuously bombarded from space by high energy sub-atomic particles. As near as we can tell, this has been the environment with which we humans have contended for our entire history on the planet.

These particles and rays racing toward our globe originate either from the sun or from other stars and enter our atmosphere traveling at a very high velocity, in some cases nearly that of the speed of light. As they enter the upper reaches of our atmosphere, they collide with atoms already present and produce secondary rays which have considerably less energy than the primary bombardment, but they are still capable of penetrating the lower layers of the atmosphere. Some of these reach the surface of the earth.

The intensity of this radiation and its power diminish rapidly with decreasing altitude below 50,000 feet, as further collisions occur with the molecules of the air. Thus, at sea level the effects of cosmic radiation are only about one-seventieth of that encountered at an altitude of 70,000 feet. It follows that as we climb in altitude, our exposure climbs as well.

The purpose of this document is to define and quantify these exposures, put them in perspective with other environmental risks, and address the issue of managing the exposure.

Definitions and Terms of Measurement

Measurements of humans' exposure to radiation are generally given as average dose equivalent rates. The term dose equivalent is a measure of the biological harmfulness of radiation and takes into account the fact that equal amounts of absorbed energy from different types of radiation are not necessarily equally harmful.

The present international unit dose equivalent is the sievert. Divisions of the sievert are used as well, so 1,000 millisieverts (mSv) equal one sievert and 1,000 microsieverts (uSv) equal one millisievert (mSv).

Ionizing Radiation

Sub-atomic particles and rays traveling at a high velocity which can theoretically cause changes to human tissue by ionizing (changing the chemistry of) cells in our bodies.

Terrestrial Radiation

That portion of ionizing radiation which comes from radioactive atoms normally present in our bodies and in the earth.

Cosmic Radiation

That portion of ionizing radiation which comes to us through the atmosphere from the sun (solar) and other stars (galactic).

Exposures

Air crew are potentially exposed to ionizing radiation in 3 general categories. Exposure varies with a number of predictable as well as some poorly predictable variables.

Terrestrial Radiation

At sea level the minimal amount of cosmic radiation which has reached the earth's surface is equivalent to approximately 0.06 microsieverts per hour (.06 uSv/Hr.) or 0.6 mSv/Yr. At higher elevations more cosmic radiation reaches the ground.

When all contributions to exposures from natural sources of radiation are taken into account, the average annual sea level dose is closer to 3.0 mSv/Yr. Considerable variation in this figure is due to terrestrial radon.

Radioactive material transported in aircraft consists mostly of pharmaceuticals used in medical diagnosis and treatment. D.O.T. Regulations are specific as to packaging and storage of such cargo in order to limit radiation levels in areas occupied by people or animals. Exposure from this sort of radiation is very low. An early Nuclear Regulatory Commission study of passenger aircraft that transport radioactive cargo in the U.S. estimated that the average annual radiation dose to crew members from that cargo amounts to less than 10 percent of the estimated total annual sea level radiation dose.

A typical chest x-ray represents an effective dose of about 0.1mSv.

Galactic Cosmic Radiation

At an altitude of 35,000 feet the dose equivalent rate from galactic cosmic radiation is about 5.0 microsieverts per hour (5.0 uSv/Hr.).

The earth's magnetic field deflects many charged particles of both solar and galactic origin that would otherwise enter the atmosphere. This shielding is most effective at the equator where the earth's magnetic lines of force are essentially parallel to the surface of the earth. This deflection decreases with increase in latitude and disappears almost entirely over the poles where the magnetic lines of force are nearly perpendicular to the surface of the earth. Therefore, at air carrier cruise altitude, the galactic radiation dose equivalent over the poles is approximately twice that as over the equator (10 uSv/Hr.). Air carrier aircraft may fly these high latitude routes between the contiguous United States and Europe or Asia.

Solar Cosmic Radiation

Although charged particles are continuously being ejected from the sun, they are usually too low in energy to contribute to the radiation level at air carrier altitudes. However, on rare and unpredictable occasions, the numbers and energies of ejected solar particles are high enough to increase substantially the dose equivalent rate at these altitudes. These are known as solar particle events.

Measurements of exposures as well as guidelines and standards for these exposures generally include all of these types of exposure to radiation; i.e., terrestrial, galactic, and solar.

Exposures are variable depending upon several factors. These include:

Flight altitude and duration at that altitude

Geographic latitude of the flight

The sun's solar cycle

In general, with a decrease in altitude from approximately 70,000 feet, the amount of galactic cosmic radiation decreases. As in the discussion above, latitude and its relationship to the protective action of the earth's magnetic field is an important factor.

Finally, the amount of cosmic radiation particles entering the atmosphere varies inversely with the approximate eleven year cycle of rise and decline of solar activity known as the solar cycle.

Obviously the first two variables (altitude/duration, and latitude) can be influenced by flight parameters while we have some predictability and no control over solar events.

Given what we know about radiation exposures in the aviation environment, there are several means of obtaining estimates of the amount of cosmic radiation received by crew members during particular flight segments. Approximations can be garnered by multiplying block hours by average exposures as described above, and for most crew members this will suffice. The average air crew dose will probably lie in the range of three to six millisieverts per year (3 to 6 mSv/Yr.), with the amount of individual radiation depending on number of flight hours, flight altitude and latitude, and solar activity.

Recent British Airways' research looking at high altitude, long duration flights gave an effective dose rate of 3.5 mSv/Yr. Slightly higher doses are recorded for Concorde crews; slightly lower, for short-haul crews.

The FAA has developed a computer software program for public use, entitled "CARI-6" that provides an estimated equivalent dose for a particular flight when certain parameters of the flight are supplied to it.

Standards and Guidelines

There are a number of national and international organizations that provide guidelines for ionizing radiation exposure limits. The Nuclear Regulatory Commission (NRC), the Environmental Protection Agency (EPA), the International Commission on Radiological Protections (ICRP), and the National Council on Radiation Protection and Measurement (NCRP), have all weighed in with standards and guidelines which are generally consistent among the groups.

The standard of radiation protection provided for members of the general public is a yearly limit of 1mSv. (ICRP-1990)

The recommended radiation exposure for workers in the nuclear industry is 20 mSv/Yr. averaged over five years (ICRP - 1991).

During pregnancy, the radiation exposure limit is recommended to be no more than 2 mSv total. In addition, recommendations include that the exposure of the unborn child not exceed 0.5 mSv in any month (excluding medical exposures) once pregnancy becomes known (NCRP - 1993).

The National Radiological Protection Board (NRPB) of the U.K. recommends that the exposure of pregnant women should be "as low as reasonably achievable" and such as to make it unlikely that the equivalent dose to the fetus will exceed 1 mSv during the remainder of the pregnancy. A recent FAA Office of Aviation Medicine report agrees with these British recommendations. (FAA-2000)

These recommended exposure limits have been established based on an attempt to keep the risk of adverse effects at a minimum. In general, exposure limits for both the individual member of the public and the occupationally exposed worker in the nuclear industry have continued to be reduced over time.

Cosmic Radiation Exposure Risks

Although the risks from high levels of radiation are well known, the effects of low level doses of radiation, such as cosmic radiation or medical tests, are more difficult to predict. These hypothetical risks of low-level radiation exposure to human beings can be broadly broken down to three general categories: cancer risk, genetic risk, and risk to the unborn child.

The risk of developing cancer is the principle health concern associated with occupational exposure to radiation. The average person on the ground has the risk of dying from cancer of about 23 percent in a lifetime. Based on increased crew member exposure to radiation, models predict that the total risk of dying from cancer for a crew member from long-haul routes for 20 years would increase from the normal 23 percent to approximately 23.3 percent. The time for a radiation-induced cancer to manifest itself is normally in the range of 10 to 50 years. We should note that research projects investigating health effects in flight crews caused by cosmic radiation exposure are relatively few and while these studies have reported some additional cancers, their results are far from definitive and are often conflicting.

With respect to genetic risk, in the general population about 2 to 3 percent of children are born with serious anatomic abnormalities. Models constructed over the past decade predict the total genetic risk of producing such a child which is attributable to radiation exposure to be approximately 75 in 1 million. Thus, the risk to the child as a result of work-related radiation exposure of a parent would be slightly less than the current general population incidence. Whether these risks are additive is not clear.

Another way of looking at risk: 1 mSv equates to a cancer risk of 1 in 20,000; a severe hereditary effects risk of 1 in 100,000.

For a fetus receiving ionizing radiation during a pregnancy, the risk of harm depends on the stage of development at the time of exposure as well as the amount of radiation. Estimates of health risks at various stages of prenatal development indicate a risk to the unborn child of approximately 11 in 10,000 of one or more serious health effects from radiation exposure above recommended limits.

Managing Exposure

Although the risks are exceedingly small, the greater the dose equivalent of radiation that a crew member occupationally receives or is exposed to, the greater the hypothetical risk of potential harmful effects. It would follow then that managing exposure to cosmic radiation may, at times, be appropriate for the individual crew member.

Table 1 demonstrates how the variables of flight altitude, latitude, and duration affect radiation exposure. For example, polar flights that are of long duration and flown at higher altitudes and latitudes receive higher annual equivalent doses than flights that are of shorter duration, lower altitude and lower latitude.

At this time, personal dosimetry (radiation badges) is an inexact science owing to problems of background sea level radiation and our inability to separate out meaningful radiation at altitude.

Monitoring one's own exposure is now possible utilizing web based calculators such as that previously described as available from the FAA. Additionally, other related web sites are available for your perusal and education.

With respect to individual consultation, United's Regional Flight Surgeons are aware of the aerospace medical issues involved and encourage you to discuss specific concerns with them. For the female crew member who may be pregnant or plans to become pregnant and wants to decrease risk to her offspring, a consultation with your obstetrician is certainly appropriate, and we are happy to consult with your private physician(s) as well.

With respect to solar particle events, systems are in place to monitor such activity but are likely to be irrelevant to our everyday operations. In the 100,000 legs flown by British Airways' Concorde, emergency descent procedures to lower altitudes have never been engaged. Various governmental agencies including the NOAA and NASA have had aircraft on standby to monitor high radiation dosages during solar events and have failed to find meaningful activity at flight altitudes.

Because solar particle events can hypothetically produce elevated radiation levels at high altitudes/latitudes, routine forecasts and alerts are sent through the FAA so that a flight in potential danger could alter its flight plan and reduce altitude to minimize exposure.

Acknowledgements

The United Airlines Medical Department would like to thank Dr. Michael Bagshaw of British Airways, Dr. David McKenas of American Airlines, Dr. Claude Thibeault of Air Canada, and Dr. Wallace Friedberg of the FAA for their input.